The present invention relates to a castable refractory material mainly used for linings of vessels such as tundishes and ladles for molten steel, etc.
Refractories are damaged mainly by melting and peeling. With respect to the melting, the castable refractory material can be provided with an improved resistance to the melting by using highly pure starting materials or by using fine powder serving to make the structures of the refractory concretes denser. However, no effective solutions have been found yet to the peeling of linings from ladles for molten steel, etc., due to mechanical spalling. The term "mechanical spalling" used herein means, for instance, cracking or peeling caused by mechanical impact to remove slag, metals or coating materials, etc., attached to the surfaces of the tundishes, etc., after casting. In addition, the linings are subjected to thermal expansion during operations, but they are restrained by outside iron shells. Accordingly, stress is likely to be generated in the refractory linings, causing the peeling of the refractory linings. Particularly, since there is a temperature gradient in the refractory linings of the melt vessels from the inner surfaces to the outer surfaces, peeling is likely to take place due to stress concentration in a region which is subjected to a temperature at which the strength of the lining materials is drastically decreased.
It may be contemplated to prevent the peeling of linings by increasing the strength of the lining materials. However, since the castable refractory material is not burned before casting, unlike refractory bricks, ceramic bonding cannot be relied on as a force to prevent the peeling.
Japanese Patent Laid-Open No. 54-113617 discloses a high-strength, unshaped refractory material comprising a refractory material based on alumina-silica, 0.5-12 weight % of an alumina cement containing 70% or more of alumina, and 1.0-8.0 weight % of fine amorphous silica flour having an apparent average diameter of 3 .mu.m or less, based on the total amount thereof. In this refractory material, the silica flour and the alumina cement are used to achieve high strength. However, sufficient strength cannot be achieved at a low temperature unless the silica flour and the alumina cement are added in such large amounts as to damage the corrosion resistance.
Japanese Patent Laid-Open No. 57-172181 discloses a castable refractory material for linings of vessels for molten metals containing 0.5-4.0 weight % of refractory clay and 0.5-2.0 weight % of ultra-fine silica flour having an average diameter of at least 0.1 .mu.m, based on the total amount thereof. In this refractory material, silica flour and alumina cement are used in desired amounts from the viewpoint of corrosion resistance. However, sufficient strength cannot be achieved yet.
Large amounts of alumina cement, etc., are used to achieve high strength in these references. This is due to the fact that sufficient consideration has not been made on the properties of alumina cement and the particle size distribution of binder-constituting materials, particularly fine refractory powder of alumina, etc.
Apart from the above, it is known that various fine refractory powders are added to increase a packing density, thereby improving physical strength. For instance, Japanese Patent Publication No. 58-33195 discloses a castable refractory material comprising a refractory material containing 5-30 weight % of ultra-fine powder having a particle size of 10 .mu.m or less without binder clay, a dispersant and an agglomeration agent such as gelatin, casein, vegetable rubber, cellulose paste, PVA, etc., the refractory material being one or more selected from the group consisting of oxides, carbides, nitrides, silicides, borides and carbonaceous materials. The oxides include alumina, magnesia, silica, etc. However, this reference fails to teach that refractory powder having different sizes should be added to castable refractory materials to achieve high strength.
To withstand a mechanical impact from outside and a stress caused by thermal expansion, it is preferable that the refractory concretes have a modulus of rupture of 120 kgf/cm.sup.2 or more and a crushing strength of 700 kgf/cm.sup.2 or more at a temperature of 110.degree. C. or higher.